1. Field of the Invention
The invention relates to a liquid fuel injection system for a direct-injection engine.
2. Background Art
A fuel injector for an internal combustion engine, such as a diesel cycle engine, has a fuel injection pump plunger that reciprocates in a plunger cylinder or bore to effect fuel delivery to nozzles for each of the working cylinders of the engine. The plunger is stroked with a frequency directly proportional to engine speed since it is driven by an engine valve camshaft. The fuel injector includes an electromagnetic solenoid actuator for a fuel control valve, which controls delivery of fuel from a high pressure pumping chamber of the injector to the fuel injection nozzles. The solenoid actuator for the valve may be under the control of a digital electronic engine controller, which distributes controlled current pulses to the actuator to effect metering of fuel from the injector to the nozzles as the injector creates pressure pulses for the injection events.
The camshaft is located in a cylinder housing for the engine where it is exposed to engine lubricating oil. Any fuel that leaks through a clearance between the plunger and the plunger cylinder or bore tends to commingle with the lubricating oil, thereby creating a lubrication oil dilution problem after an extended operating period.
It is possible to reduce leakage past the plunger by reducing the dimensional clearance between the plunger and the plunger cylinder or bore. A reduction in the dimensional clearance, however, increases the risk of plunger seizure. This creates a design problem because mechanical friction losses and increased wear, especially in those instances when the fuel temperature varies throughout a relatively wide temperature range. Furthermore, precise machining required for close tolerance fits between the plunger and the plunger cylinder or bore increases manufacturing costs, which would make such designs impractical for high volume manufacturing operations.
A reduction in lubrication oil dilution can be achieved also by increasing the length of the plunger, thereby increasing the leak flow path length. It has been found, however, that this results only in a moderate decrease in leakage. Further, this would require an undesirable increase in the overall dimensions of the injector. Such increased dimensions of the injector would make it impractical in some commercial engine applications because of packaging constraints as well as cost penalties.
The present invention is adapted particularly for use with a xe2x80x9cdual railxe2x80x9d injector design. That is, fuel is delivered to the injector through a fuel supply rail or passage from a low pressure fuel supply pump. Fuel that is not distributed to the nozzles, which is referred to as spill fuel, is returned to the inlet side of the fuel pump through a separate rail or return flow passage. It is an objective of the invention to reduce engine oil dilution in such a dual rail injector. This is done by decreasing leakage of fuel past the injector plunger into the lubrication oil circuit. This isolates the leak flow path from the region of the engine occupied by the camshaft that drives the injector plunger.
The injector of the invention comprises a fuel pump body with a cylinder that receives the injector pump plunger. A plunger spring normally urges the plunger to a retracted position. The plunger is driven during its working stroke by the engine camshaft.
The plunger and the cylinder or bore define a high pressure pumping chamber that communicates with an injector nozzle through a high pressure fuel delivery passage. Typically, the pressure may be about 20K psi. The high pressure passage is intersected by a pump control valve. Fuel is supplied to the control valve and to the pumping chamber of the injector by a fuel supply pump. The control valve opens and closes the fuel flow path through the high pressure fuel delivery passage in accordance with commands transmitted to a control valve solenoid actuator by an engine controller. The valve is opened and closed at the desired frequency for the injection pulses.
Separate fuel supply and return passages communicate with the control valve and with the pumping chamber. A separate leak-off passage communicates with the injector body and extends to the plunger cylinder at a location intermediate the full stroke position of the plunger and the full retracted position of the plunger. The leak-off passage communicates with a fuel tank, which is under zero gauge pressure. The leak flow path is defined by a predetermined clearance between the plunger and the plunger cylinder. It communicates with the leak-off passage so that leakage fuel will return to the tank rather than flow to the region of the camshaft in the engine cylinder housing. The fuel supply and return circuit is independent of the lubrication oil for the engine so that oil dilution is eliminated or substantially reduced. This increases the durability of the fuel injector and reduces maintenance costs for the engine.
In accordance with one embodiment of the invention, the fuel supply passage communicates with the injector pump body and with an internal passage that communicates with the chamber occupied by the flow control valve. A separate flow return passage in the injector pump body, which sometimes is referred to as a spill passage, communicates with an internal groove that in turn communicates with the return passage. Typically, the spill passage within the injector pump body may have a pressure of about 2K psi.
In a first alternate embodiment of the invention, the return passage is connected to the injector pump body at the upper end of the body adjacent the control valve.
In a second alternate embodiment of the invention, the return passage communicates with the flow control valve through an internal passage in the injector pump body and the supply passage communicates with the region of an actuator for the control valve.
In a third alternate embodiment of the invention, the leak-off passage extends generally in the direction of the axis of plunger cylinder in the pump body. The pump body is mounted in a sleeve in the engine cylinder housing. A leak-off passage fitting on the pump body, as well as a fuel supply passage fitting, are conveniently located externally of the engine cylinder housing.
In each of the embodiments, the leak-off passage is entirely independent of the supply passage and the return passage and is subjected to zero gauge pressure.